Index fossils are fossils which are characteristic of a given layer in the stratigraphy. These are widely used to study the biostratigraphy. Suitable index fossils are fossils that have a limited distribution in time, have a large range and are not rare. The best index fossils have evolved quickly and have a short life span. On the basis of index fossils, layers can quickly and easily be dated. For more extensive dating, it is important to combine various dating methods.